Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
In a large mixing bowl, combine the flour, sugar, baking powder, and salt. Add the softened butter, milk, eggs, and vanilla extract. Beat on medium speed for about 2 minutes until well combined.
Pour the batter into the prepared baking pan and spread it evenly. Bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Allow the cake to cool in the pan for about 10 minutes, then poke holes all over the top using a fork or skewer.
In a separate bowl, dissolve the strawberry Jell-O in 1 cup of boiling water, stirring until fully dissolved. Add ½ cup of cold water and mix well. Pour half of the strawberry Jell-O mixture over the cake, ensuring it seeps into the holes.
In another bowl, dissolve the blue raspberry Jell-O in 1 cup of boiling water, stirring until fully dissolved. Add ½ cup of cold water and mix well. Pour the blue raspberry Jell-O mixture over the cake, allowing it to seep into the holes as well.
Cover the cake with plastic wrap and refrigerate for at least 4 hours, or overnight for best results.
Once chilled, spread the whipped topping evenly over the cake. Garnish with fresh strawberries and blueberries, and add whipped cream if desired.